Electrodes formed by coating active electrode materials containing platinum group metal oxides on substrates made of corrosion-resistant metals represented by titanium are known as excellent insoluble electrodes and have been put to practical use. Such electrodes are now extensively used industrially in various electrochemical fields especially as chlorine-evolving anodes in the electrolysis of common salt water. Although various improvements of these kinds of electrodes have been made in electrochemical properties and physical properties including durability, the improvements so far made are not satisfactory. In particular, where electrolysis is conducted using as an electrolyte a solution containing sulfuric acid or a salt thereof, there is a problem in that the anode used has a short lifetime because an oxygen-evolving reaction takes place mainly at the anodes and hence, the electrode is exposed to an extremely severe environment. It is thought that the principal cause for this is that together with the erosion of the active electrode material coating, oxidation of both the coating and the substrate metal, but mostly of the substrate metal occurs at the interface between the substrate and the coating to form a poorly conductive oxide etc., which accumulates at the interface, and as a result, the electrode becomes passivated or the coating peels off.
For improving the poor durability of such electrodes, various means have been proposed, such as a method to provide intermediate layers of various materials between the substrates and electrode coatings to thereby protect the substrates. (See, for example, U.S. Pat. Nos. 3,775284, 4,468,416, 4,471,006, 4,481,097, 4,584,084, 4,581,117 and 4,765,879, and GB 2192008A)
With recent developments in the electrochemical industry, however, there has been a strong demand for improvements in product quality, production efficiency, etc., so that electrolysis is conducted under severe conditions, such as diversified electrolytes, increased current densities, heightened electrolysis temperatures, etc. Therefore, the electrodes used are desired to have further improved durability and other properties.